Variable-Size Dado Fixture for a Router
Here’s an adjustable jig that allows you to rout a dado of any width, even ones that don’t fit standard bit sizes. It consists of a fixed base and a sliding base that meet at a 10° angle. Each base has a 2-in.-high cleat for clamping. When building the fixture, first make the sliding base a little wider at the fence and trim it to size with the first router cut using the bit you will use as a standard.
To align and cut the dado, first clamp the two bases together. Place the router bit edge of the jig directly on the cut line, clamp the fixed base to the workpiece, and rout the first pass of the dado. Now remove the cleat clamps and adjust the sliding base for the second pass by moving it up or down, effectively moving the router cut left or right of the previous setting. After positioning the sliding base, reclamp across the cleats and make the second pass.
The concept can be applied to any size jig. In my case, the jig was 50 in. long and roughly 18 in. wide. You can place index marks on the clamping cleats for future reference.